This invention relates to a method of producing fused silica of very high purity, and more particularly to a method of increasing the refractoriness of the silica by doping with controlled minute amounts of a selected metal oxide in conjunction with elemental silicon.
High purity fused silica can be produced from a number of different materials, and by various different procedures. A commercially used process involves vapor phase oxidation of silicon tetrachloride (SiCl.sub.4) to silica. This process is generally described for example in U.S. Pat. No. 2,272,342 granted Feb. 10, 1942 to J. F. Hyde. Various other halides, silanes, and other volatile silicon compounds may be employed in lieu of the chloride. Alternatively, combustion processes such as are described in U.S. Pat. Nos. 2,823,982, granted Feb. 18, 1958 to O. Saladin et al., and No. 3,698,936, granted Oct. 17, 1972 to H. J. Moltzan may be employed. Also, hydrolytic processes starting with organic silicates are well known. The main factor in achieving a high purity level with any of these processes is the purity of the particular starting material selected.
The purity factor is very important in certain products such as the silica crucibles used in melting and drawing elemental silicon for semiconductor use. For such purposes alkali metal ions must be below ten (10) parts per million (ppm), and the transition metals must be below one ppm.
In addition to a high purity level, fused silica is frequently required to have a relatively high refractoriness or heat resistance. This characteristic is commonly identified with reference to the viscosity-temperature relationship of the ultimate vitrified silica. Glass annealing and strain points are commonly recognized and readily measured indicia of refractoriness or heat resistance.